FIGS. 2(a) and (b) show a prior art crystal growth method for producing a light emission element, wherein FIG. 2(a) shows a cross-sectional structure of semiconductor layers before growing a double heterojunction structure, and FIG. 2(b) shows a cross-sectional structure of semiconductor layers after growing a double heterojunction structure.
In these figures, the reference numeral 1 designates a GaAs substrate, the numeral 2 designates a GaAs first buffer layer, the numeral 5 designates an A1GaInP first cladding layer, the numeral 6 designates a GaInP active layer, the numeral 7 designates an A1GaInP second cladding layer, and the numeral 8 designates a GaAs contact layer.
Usually, the growth of an A1GaInP series double heterojunction structure by the use of an MO-CVD method is conducted in accordance with the following production process.
At first, as shown in FIG. 2(a), a GaAs first buffer layer 2 is grown on a GaAs substrate 1. Next, as shown in FIG. 2(b), an A1GaInP first cladding layer 5, a GaInP active layer 6, an A1GaInP second cladding layer 7, and a GaAs contact layer 8 are successively grown on the GaAs first buffer layer 2.
When the growth of an A1GaInP series double heterostructure is conducted by such a process, many defects occur in the growing layer because impurities such as oxygen or moisture enter the reaction tube when a wafer is mounted on a susceptor. These impurities are adsorbed on the susceptor surface or the reaction tube wall. It is difficult to remove these impurities even by hydrogen purging or the like.
As means for reducing air contamination when mounting the wafer in an MO-CVD method, a so-called air lock system is usually employed.
FIG. 3 is a schematic diagram showing an MO-CVD apparatus provided with such an air lock system.
In FIG. 3, the reference numeral 9 designates a
gas introduction aperture, the numeral 10 designates a susceptor, the numeral 11 designates a wafer, the numeral 12 designates an exhaust aperture, the numeral 13 designates a gate valve, the numeral 14 designates a wafer exchange room, the numeral 15 designates a wafer exchange instrument, and the numeral 16 designates a reaction tube.
In this MO-CVD apparatus provided with an air lock system, at first the wafer 11 is introduced into the wafer exchange room 14, and the introduced wafer 11 is sufficiently purged therein. Next, the gate valve 13 is opened, the susceptor 10 and wafer exchange instrument 15 are moved, and the wafer 11 is mounted onto the susceptor 10. Thus, the mounting of the wafer 11 is conducted without the reaction tube 16 and the susceptor 10 being exposed to air.
In this air lock system employed in the prior art crystal growth method, however, the configuration and number of wafers 11 are restricted because the mounting of the wafer 11 is conducted by a machine, and it takes a relatively long time for the apparatus to equilibrate after mounting or removing a wafer. Furthermore, impurities attached to the wafer 11 themselves cannot be removed. The air lock system apparatus is also expensive.